If a plant has photosynthesised it will test positive for starch.
The starch test:
- Place the leaf in boiling water for 30 seconds – this kills it, stopping any further chemical reactions.
- Place the leaf in boiling ethanol – removes the chlorophyll making the leaf paler in colour.
- Dip the leaf in water – this softens it.
- Spread the leaf onto a white tile and add iodine to test for the presence of starch.
- If starch is present the iodine will change from yellow-brown to blue-black.
- If starch is absent the iodine will remain yellow-brown.
- As well as wearing goggles throughout the experiment, ensure that the ethanol is not exposed to a naked flame during step 2, as it is highly flammable.
Destarching a plant:
- Before carrying out any photosynthesis practical it is necessary to destarch the plant by placing it in the dark for at least 48 hours. During this time any starch will be removed or used.
- This step is important so you can be positive that any starch present at the end of the experiment has been produced during the experiment.
- You can check if a plant has been fully destarched by testing a leaf for starch. If the iodine remains yellow-brown, all the starch has successfully been removed.